Laser beam shaping for foil-based metallization of solar cells

ABSTRACT

Approaches for foil-based metallization of solar cells and the resulting solar cells are described. For example, a method of fabricating a solar cell involves locating a metal foil above a plurality of alternating N-type and P-type semiconductor regions disposed in or above a substrate. The method also involves laser welding the metal foil to the alternating N-type and P-type semiconductor regions. The method also involves patterning the metal foil by laser ablating through at least a portion of the metal foil at regions in alignment with locations between the alternating N-type and P-type semiconductor regions. The laser welding and the patterning are performed at the same time.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.16/686,016, filed on Nov. 15, 2019, which is a continuation of U.S.patent application Ser. No. 15/454,890, filed on Mar. 9, 2017, now U.S.Pat. No. 10,535,785, issued Jan. 14, 2020, which is a continuation ofU.S. patent application Ser. No. 14/578,334, filed on Dec. 19, 2014, nowU.S. Pat. No. 9,620,661, issued on Apr. 11, 2017, the entire contents ofwhich are hereby incorporated by reference herein.

TECHNICAL FIELD

Embodiments of the present disclosure are in the field of renewableenergy and, in particular, include approaches for foil-basedmetallization of solar cells and the resulting solar cells.

BACKGROUND

Photovoltaic cells, commonly known as solar cells, are well knowndevices for direct conversion of solar radiation into electrical energy.Generally, solar cells are fabricated on a semiconductor wafer orsubstrate using semiconductor processing techniques to form a p-njunction near a surface of the substrate. Solar radiation impinging onthe surface of, and entering into, the substrate creates electron andhole pairs in the bulk of the substrate. The electron and hole pairsmigrate to p-doped and n-doped regions in the substrate, therebygenerating a voltage differential between the doped regions. The dopedregions are connected to conductive regions on the solar cell to directan electrical current from the cell to an external circuit coupledthereto.

Efficiency is an important characteristic of a solar cell as it isdirectly related to the capability of the solar cell to generate power.Likewise, efficiency in producing solar cells is directly related to thecost effectiveness of such solar cells. Accordingly, techniques forincreasing the efficiency of solar cells, or techniques for increasingthe efficiency in the manufacture of solar cells, are generallydesirable. Some embodiments of the present disclosure allow forincreased solar cell manufacture efficiency by providing novel processesfor fabricating solar cell structures. Some embodiments of the presentdisclosure allow for increased solar cell efficiency by providing novelsolar cell structures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1F illustrate cross-sectional views of various stages in thefabrication of a solar cell using foil-based metallization, inaccordance with an embodiment of the present disclosure, wherein:

FIG. 1A illustrates a stage in solar cell fabrication followingformation of alternating N-type and P-type semiconductor regions(emitter regions) formed above a portion of a back surface of asubstrate of a solar cell;

FIG. 1B illustrates the structure of FIG. 1A following optionalformation of a paste between adjacent ones of the alternating N-type andP-type semiconductor regions;

FIG. 1C illustrates the structure of FIG. 1B following optional curingof the paste to form non-conductive material regions in alignment withlocations between the alternating N-type and P-type semiconductorregions;

FIG. 1D illustrates the structure of FIG. 1C following optionalformation of a plurality of metal seed material regions to provide ametal seed material region on each of the alternating N-type and P-typesemiconductor regions;

FIG. 1E illustrates the structure of FIG. 1D following locating of ametal foil with the alternating N-type and P-type semiconductor regions;and

FIG. 1F illustrates the structure of FIG. 1E following laser ablatingthrough the metal foil in alignment with the locations between thealternating N-type and P-type semiconductor regions to laser weld themetal foil with the alternating N-type and P-type semiconductor regionsand to isolate regions of remaining metal foil in alignment with thealternating N-type and P-type semiconductor regions.

FIG. 2 is a flowchart listing operations in a method of fabricating asolar cell as corresponding to FIGS. 1A-1F, in accordance with anembodiment of the present disclosure.

FIG. 3 illustrates a cross-sectional views of another solar cell havingfoil-based metallization, in accordance with another embodiment of thepresent disclosure.

FIG. 4 is a plot of laser intensity as a function of distanceillustrating a cross-sectional view of a spatially shaped laser beamhaving a beam shape with an inner region of lower intensity and an outerregion of higher intensity, in accordance with an embodiment of thepresent disclosure.

FIGS. 5A and 5B illustrate various processing operations in a method offabricating a solar cell using the laser beam profile of FIG. 4, inaccordance with an embodiment of the present disclosure.

FIG. 6 is a schematic illustrating a cross-sectional view of a spatiallyshaped laser beam having a beam shape with an inner region of higherintensity and an outer region of lower intensity, in accordance with anembodiment of the present disclosure.

FIGS. 7A and 7B illustrate cross-sectional views of various stages inthe fabrication of a solar cell using foil-based metallization, inaccordance with another embodiment of the present disclosure.

DETAILED DESCRIPTION

The following detailed description is merely illustrative in nature andis not intended to limit the embodiments of the subject matter or theapplication and uses of such embodiments. As used herein, the word“exemplary” means “serving as an example, instance, or illustration.”Any implementation described herein as exemplary is not necessarily tobe construed as preferred or advantageous over other implementations.Furthermore, there is no intention to be bound by any expressed orimplied theory presented in the preceding technical field, background,brief summary or the following detailed description.

This specification includes references to “one embodiment” or “anembodiment.” The appearances of the phrases “in one embodiment” or “inan embodiment” do not necessarily refer to the same embodiment.Particular features, structures, or characteristics may be combined inany suitable manner consistent with this disclosure.

Terminology. The following paragraphs provide definitions and/or contextfor terms found in this disclosure (including the appended claims):

“Comprising.” This term is open-ended. As used in the appended claims,this term does not foreclose additional structure or steps.

“Configured To.” Various units or components may be described or claimedas “configured to” perform a task or tasks. In such contexts,“configured to” is used to connote structure by indicating that theunits/components include structure that performs those task or tasksduring operation. As such, the unit/component can be said to beconfigured to perform the task even when the specified unit/component isnot currently operational (e.g., is not on/active). Reciting that aunit/circuit/component is “configured to” perform one or more tasks isexpressly intended not to invoke 35 U.S.C. § 112, sixth paragraph, forthat unit/component.

“First,” “Second,” etc. As used herein, these terms are used as labelsfor nouns that they precede, and do not imply any type of ordering(e.g., spatial, temporal, logical, etc.). For example, reference to a“first” solar cell does not necessarily imply that this solar cell isthe first solar cell in a sequence; instead the term “first” is used todifferentiate this solar cell from another solar cell (e.g., a “second”solar cell).

“Coupled”—The following description refers to elements or nodes orfeatures being “coupled” together. As used herein, unless expresslystated otherwise, “coupled” means that one element/node/feature isdirectly or indirectly joined to (or directly or indirectly communicateswith) another element/node/feature, and not necessarily mechanically.

In addition, certain terminology may also be used in the followingdescription for the purpose of reference only, and thus are not intendedto be limiting. For example, terms such as “upper”, “lower”, “above”,and “below” refer to directions in the drawings to which reference ismade. Terms such as “front”, “back”, “rear”, “side”, “outboard”, and“inboard” describe the orientation and/or location of portions of thecomponent within a consistent but arbitrary frame of reference which ismade clear by reference to the text and the associated drawingsdescribing the component under discussion. Such terminology may includethe words specifically mentioned above, derivatives thereof, and wordsof similar import.

“Inhibit”—As used herein, inhibit is used to describe a reducing orminimizing effect. When a component or feature is described asinhibiting an action, motion, or condition it may completely prevent theresult or outcome or future state completely. Additionally, “inhibit”can also refer to a reduction or lessening of the outcome, performance,and/or effect which might otherwise occur. Accordingly, when acomponent, element, or feature is referred to as inhibiting a result orstate, it need not completely prevent or eliminate the result or state.

Approaches for foil-based metallization of solar cells and the resultingsolar cells are described herein. In the following description, numerousspecific details are set forth, such as specific laser spatial profilesand process flow operations, in order to provide a thoroughunderstanding of embodiments of the present disclosure. It will beapparent to one skilled in the art that embodiments of the presentdisclosure may be practiced without these specific details. In otherinstances, well-known fabrication techniques, such as lithography andpatterning techniques, are not described in detail in order to notunnecessarily obscure embodiments of the present disclosure.Furthermore, it is to be understood that the various embodiments shownin the figures are illustrative representations and are not necessarilydrawn to scale.

Disclosed herein are methods of fabricating solar cells, and theresulting solar cells. In one embodiment, a method of fabricating asolar cell involves locating a metal foil above a plurality ofalternating N-type and P-type semiconductor regions disposed in or abovea substrate. The method also involves laser welding the metal foil tothe alternating N-type and P-type semiconductor regions. The method alsoinvolves patterning the metal foil by laser ablating through at least aportion of the metal foil at regions in alignment with locations betweenthe alternating N-type and P-type semiconductor regions. The laserwelding and the patterning are performed at the same time.

In another embodiment, a method of fabricating a solar cell involveslocating a metal foil above a plurality of alternating N-type and P-typesemiconductor regions disposed in or above a substrate. The method alsoinvolves impinging an incident laser beam on the metal foil, theincident laser beam having a beam shape with an inner region of lowerintensity and an outer region of higher intensity, the inner region andthe outer region relative to a central axis of the incident laser beam.The method also involves laser welding the metal foil to the alternatingN-type and P-type semiconductor regions with the inner region of theincident laser beam. The method also involves patterning the metal foilby laser ablating with the outer region of the incident laser beamthrough at least a portion of the metal foil at regions in alignmentwith locations between the alternating N-type and P-type semiconductorregions.

In another embodiment, a method of fabricating a solar cell involveslocating a metal foil above a plurality of alternating N-type and P-typesemiconductor regions disposed in or above a substrate. The method alsoinvolves impinging an incident laser beam on the metal foil, theincident laser beam having a beam shape with an inner region of higherintensity and an outer region of lower intensity, the inner region andthe outer region relative to a central axis of the incident laser beam.The method also involves laser welding the metal foil to the alternatingN-type and P-type semiconductor regions with the outer region of theincident laser beam. The method also involves patterning the metal foilby laser ablating with the inner region of the incident laser beamthrough at least a portion of the metal foil at regions in alignmentwith locations between the alternating N-type and P-type semiconductorregions.

One or more embodiments described herein are directed to metal (such asaluminum) based metallization for solar cells. As a generalconsideration, back contact solar cells typically require patternedmetal of two types of polarity on the backside of the solar cell. Wherepre-patterned metal is not available due to cost, complexity, orefficiency reasons, low cost, low materials processing of a blanketmetal often favors laser-based pattern approaches. In an embodiment, analuminum metallization process for interdigitated back contact (IBC)solar cells is disclosed. In an embodiment, a M2-M1 process isreferenced, where the M2 layer may be fabricated from a metal foil,while the M1 layer is a metal layer (which may be referred to as a seedlayer) formed on a portion of a solar cell.

To provide context, in applying a foil based metallization for thebackside junction of a solar cell, it may be challenging to fabricate afinger (contacts) pattern from the foil whether before or after anassociated conduction bonding between the cell and the foil. Onepatterning attempted involved using a pulsed laser to ablate the foil inbetween two different polarity fingers until they were electricallyisolated. However, such “direct laser patterning” has shown littleprocessing window advantages. There have also been attempts to introducedamage absorbing layers between the M2 and the cell during thepatterning, or the use of ablating the majority thickness of the foiland using chemical etching for the complete electrical isolation, butadvances in patterning technology are still needed. In one or moreembodiments described herein, laser induced damage is avoided duringpatterning, and a number of laser exposure operations may be reduced.

Accordingly, embodiments described herein provide one or more approachesfor patterning a major current carrying layer, or M2, during a laserwelding operation using a spatially shaped beam profile. In oneembodiment, a fabrication process involves laser bonding and mechanicalfoil isolation operations to minimize thermal and optical damage duringa patterning operation while allowing high accuracy of alignment betweenthe bonding and the patterning.

In an embodiment, different strengths of adhesion among foil (M2) bondedto a vapor deposited thin seed metal (M1) and, hence, to the underlyingdevice wafer, are achieved depending on bonding method. Furthermore,different types of failure modes are observed during adhesion testing.For laser bonding, adhesion can depend on the laser fluence (energy perfocused area). At lower fluences, the adhesion between M1 and M2 is tooweak and the M2 detaches easily. As the laser fluence increases, theadhesion by the welding between the foil and the underlying M1 seedlayer becomes strong enough to tear the foil during the adhesion test.When the laser fluence becomes even higher, the underlying M1 layerbecomes affected and the M1-device wafer bonding is broken before thefoil is torn off in a peeling test. To take advantage of such differentmodes of tearing, in one embodiment, a spatially shaped laser beam isused during the laser bonding process. The laser beam can have higherintensity (M1 tearing range) at the outer region and lower intensity (M2tearing range) on the inside, such that after the welding, the foil (M2)can be torn off along with the M1, while leaving the M2/M1 region underthe weld intact.

In an illustrative example of processing that may benefit from aspatially shaped maser beam process, a laser grooving and laser weldingapproach provides a new electrode patterning method for interdigitatedback contact solar cells based on the laser patterning and welding of analuminum (Al) foil to form an inter-digitated pattern of contactfingers. Embodiments of such an approach can be implemented to provide adamage-free method to patterning and welding an Al foil on the wafer,avoiding complex alignment and/or masking processes. As an example,FIGS. 1A-1F illustrate cross-sectional views of various stages in thefabrication of a solar cell using foil-based metallization, inaccordance with an embodiment of the present disclosure. FIG. 2 is aflowchart listing operations in a method of fabricating a solar cell ascorresponding at least to some of FIGS. 1A-1F, in accordance with anembodiment of the present disclosure.

FIG. 1A illustrates a stage in solar cell fabrication followingformation of emitter regions formed above a portion of a back surface ofa substrate of a solar cell. Referring to FIG. 1A and corresponding to aportion of operation 202 of flowchart 200, a plurality of alternatingN-type and P-type semiconductor regions are formed above a substrate. Inparticular, a substrate 100 has disposed there above N-typesemiconductor regions 104 and P-type semiconductor regions 106 disposedon a thin dielectric material 102 as an intervening material between theN-type semiconductor regions 104 or P-type semiconductor regions 106,respectively, and the substrate 100. The substrate 100 has alight-receiving surface 101 opposite a back surface above which theN-type semiconductor regions 104 and P-type semiconductor regions 106are formed.

In an embodiment, the substrate 100 is a monocrystalline siliconsubstrate, such as a bulk single crystalline N-type doped siliconsubstrate. It is to be appreciated, however, that substrate 100 may be alayer, such as a multi-crystalline silicon layer, disposed on a globalsolar cell substrate. In an embodiment, the thin dielectric layer 102 isa tunneling silicon oxide layer having a thickness of approximately 2nanometers or less. In one such embodiment, the term “tunnelingdielectric layer” refers to a very thin dielectric layer, through whichelectrical conduction can be achieved. The conduction may be due toquantum tunneling and/or the presence of small regions of directphysical connection through thin spots in the dielectric layer. In oneembodiment, the tunneling dielectric layer is or includes a thin siliconoxide layer. In other embodiments, N-type and P-type emitter regions areformed in the substrate itself, in which case distinct semiconductorregions (such as regions 104 and 106) and the dielectric layer 102 wouldnot be included.

In an embodiment, the alternating N-type and P-type semiconductorregions 104 and 106, respectively, are formed polycrystalline siliconformed by, e.g., using a plasma-enhanced chemical vapor deposition(PECVD) process. In one such embodiment, the N-type polycrystallinesilicon emitter regions 104 are doped with an N-type impurity, such asphosphorus. The P-type polycrystalline silicon emitter regions 106 aredoped with a P-type impurity, such as boron. As is depicted in FIG. 1A,the alternating N-type and P-type semiconductor regions 104 and 106 mayhave trenches 108 formed there between, the trenches 108 extendingpartially into the substrate 100. Additionally, although not depicted,in one embodiment, a bottom anti-reflective coating (BARC) material orother protective layer (such as a layer amorphous silicon) is formed onthe alternating N-type and P-type semiconductor regions 104 and 106.

In an embodiment, the light receiving surface 101 is a texturizedlight-receiving surface, as is depicted in FIG. 1A. In one embodiment, ahydroxide-based wet etchant is employed to texturize the light receivingsurface 101 of the substrate 100 and, possibly, the trench 108 surfacesas is also depicted in FIG. 1A. It is to be appreciated that the timingof the texturizing of the light receiving surface may vary. For example,the texturizing may be performed before or after the formation of thethin dielectric layer 102. In an embodiment, a texturized surface may beone which has a regular or an irregular shaped surface for scatteringincoming light, decreasing the amount of light reflected off of thelight receiving surface 101 of the solar cell. Referring again to FIG.1A, additional embodiments can include formation of a passivation and/oranti-reflective coating (ARC) layers (shown collectively as layer 112)on the light-receiving surface 101. It is to be appreciated that thetiming of the formation of passivation and/or ARC layers may also vary.

FIG. 1B illustrates a stage in solar cell fabrication following optionalformation of a paste between adjacent ones of the alternating N-type andP-type semiconductor regions. Referring to FIG. 1B, regions of a pastematerial 120 are formed between adjacent ones of the alternating N-typeand P-type semiconductor regions 104 and 106. In embodiments wheretrenches 108 have been formed, the paste 120 is formed within thetrenches 108, as is depicted in FIG. 1B.

In an embodiment, the regions of the paste material 120 are formed byscreen printing the paste. In one such embodiment, the screen printingpermits forming the regions of the paste material 120 in a pattern thatleaves exposed surfaces of the alternating N-type and P-typesemiconductor regions 104 and 106, as is depicted in FIG. 1B. In anembodiment, the regions of the paste material 120 are formed from apaste suitable for forming a non-conductive region of a solar cell. Inone such embodiment, the paste includes a binder, an opacifying pigment,and an organic medium mixed with the binder and the opacifying pigment.In an embodiment, with reference again to the paste 120, the paste has acure temperature of or less than approximately 450 degrees Celsius.

FIG. 1C illustrates a stage in solar cell fabrication following optionalcuring of the paste. Referring to FIG. 1C, the regions of paste material120 are cured to form non-conductive material regions 122 in alignmentwith locations between the alternating N-type and P-type semiconductorregions.

In an embodiment, curing the paste 120 to form the non-conductivematerial regions 122 involves heating the paste but limited to atemperature of or less than approximately 450 degrees Celsius. Inanother embodiment, curing the paste 120 to form the non-conductivematerial regions 122 involves exposing to ultra-violet (UV) radiation,or a combination of heating and exposing to UV radiation. In anembodiment, upon curing, substantially all of the organic medium of thepaste is removed, while substantially all of the binder and theopacifying pigment of the paste are retained. In one such embodiment,the binder of the paste is an inorganic binder, and the curing involvesconverting the inorganic binder to a rigid inorganic matrix of thenon-conductive material regions 122.

FIG. 1D illustrates a stage in solar cell fabrication following optionalformation of a metal layer on the structure of FIG. 1C. Referring toFIG. 1D, a metal layer (which may be referred to as a metal seed layer,or M1 layer, for the solar cell) is formed and depicted as layer 124. Inan embodiment, the metal layer 124 can be viewed as providing aplurality of metal seed material regions, with a metal seed materialregion on each of the alternating N-type and P-type semiconductorregions 104 and 106. That is, even though a single, uninterrupted layermay be formed on both the non-conductive material regions 122 and thealternating N-type and P-type semiconductor regions 104 and 106, regionswhere the metal layer 124 contact the alternating N-type and P-typesemiconductor regions 104 and 106 may be viewed as a corresponding metalseed regions. In alternative embodiments, a patterned metal layer isformed to provide corresponding metal seed regions. In either case, inan embodiment, the metal layer 124 is an aluminum layer. In oneparticular embodiment, the aluminum layer has a thickness approximatelyin the range of 0.3 to 20 microns and includes aluminum in an amountgreater than approximately 97 atomic % and silicon in an amountapproximately in the range of 0-2 atomic %. In another particularembodiment, the aluminum layer is formed by physical vapor deposition toa thickness less than approximately 1 micron. In other embodiments, themetal layer 124 includes a metal such as, but not limited to, nickel,silver, cobalt or tungsten.

FIG. 1E illustrates a stage in solar cell fabrication followingpositioning (or locating or fitting up) of a metal foil on the structureof FIG. 1D. Referring to FIG. 1E and again to corresponding operation202 of flowchart 200, a metal foil 126 is located with the alternatingN-type and P-type semiconductor regions 104 and 106. In the embodimentshown, the metal foil 126 is placed on the metal layer 124.

In an embodiment, metal foil 126 is an aluminum (Al) foil having athickness approximately in the range of 5-100 microns and, preferably, athickness of less than approximately in the range of 50 microns. In oneembodiment, the Al foil is an aluminum alloy foil including aluminum andsecond element such as, but not limited to, copper, manganese, silicon,magnesium, zinc, tin, lithium, or combinations thereof. In oneembodiment, the Al foil is a temper grade foil such as, but not limitedto, F-grade (as fabricated), 0-grade (full soft), H-grade (strainhardened) or T-grade (heat treated).

It is to be appreciated that, in accordance with another embodiment, aseedless (124 metal layer-free) approach may be implemented. In such anapproach, the metal foil 126 is ultimately adhered directly to thematerial of the alternating N-type and P-type semiconductor regions 104and 106, as is described in greater detail below in association withFIG. 3. For example, in one embodiment, the metal foil 126 is locatedwith (and then ultimately adhered directly to) alternating N-type andP-type polycrystalline silicon regions.

FIG. 1F illustrates a stage in solar cell fabrication followingpatterning of the metal foil of the structure of FIG. 1E. Referring toFIG. 1F and corresponding operation 206 of flowchart 200, a laserablating process 130 is performed through the metal foil 126 inalignment with the locations between the alternating N-type and P-typesemiconductor regions 104 and 106 to isolate regions of remaining metalfoil 126 in alignment with the alternating N-type and P-typesemiconductor regions 104 and 106. In an optional embodiment, thenon-conductive material regions 122 act as a laser stop during the laserablating 130, as is depicted in FIG. 1F.

Referring again to FIG. 1F and to corresponding operation 204 offlowchart 200, the metal foil 126 is also laser welded to the metallayer 124. The laser welding forms weld regions 128. In one suchembodiment, the weld regions 128 are formed at locations above thealternating N-type and P-type semiconductor regions 104 and 106, as isdepicted in FIG. 1F. In an embodiment, as described in association withFIG. 1D, a metal seed material region (e.g., as metal layer 124) isprovided on each of the alternating N-type and P-type semiconductorregions 104 and 106. In that embodiment, the metal foil 126 is welded tothe alternating N-type and P-type semiconductor regions 104 and 106 bylaser welding the metal foil 126 the plurality of metal seed materialregions 124 via weld regions 128, as is depicted in FIG. 1F.

In an embodiment, referring again to FIG. 1F and collectively tooperations 204 and 206 of flowchart 200, the laser welding and the foilpatterning are performed at the same time. In one such embodiment, thelaser ablating through at least a portion of the metal foil 126(patterning) involves laser ablating through an entire thickness of themetal foil, as is depicted in FIG. 1F. In another embodiment, the laserablating through at least a portion of the metal foil 126 (patterning)involves laser ablating through only a portion of the metal foil 126,the portion having a thickness approximately in the range of 80-99% ofan entire thickness of the metal foil 126, an example of which isdescribed below in association with FIG. 7B. In a specific example ofthe latter embodiment, subsequent to patterning the metal foil 126, theremaining metal foil is etched to isolate regions of the remaining metalfoil in alignment with the alternating N-type and P-type semiconductorregions. In another specific example, of the latter embodiment,subsequent to patterning the metal foil 126, the remaining metal foil isanodized to isolate regions of the remaining metal foil in alignmentwith the alternating N-type and P-type semiconductor regions. Bothexamples are described in greater detail below.

In an embodiment, referring again to FIG. 1F, simultaneous or nearsimultaneous laser welding and laser patterning of the metal foil 126 isperformed by impinging an incident laser beam on the metal foil 126. Ina first such embodiment, the incident laser beam has a beam shape withan inner region of lower intensity and an outer region of higherintensity, the inner region and the outer region relative to a centralaxis of the incident laser beam, as is described in association withFIGS. 4, 5A and 5B. In that embodiment, laser welding of the metal foil126 to the alternating N-type and P-type semiconductor regions 104/106is achieved with the inner region of the incident laser beam. Meanwhile,patterning of the metal foil 126 is achieved by laser ablating with theouter region of the incident laser beam through at least a portion ofthe metal foil 126 at regions in alignment with locations between thealternating N-type and P-type semiconductor regions 104 and 106.

In a second such embodiment, the incident laser beam has a beam shapewith an inner region of higher intensity and an outer region of lowerintensity, the inner region and the outer region relative to a centralaxis of the incident laser beam, as is described in association withFIG. 6. In that embodiment, laser welding of the metal foil 126 to thealternating N-type and P-type semiconductor regions 104/106 is achievedwith the outer region of the incident laser beam. Meanwhile, patterningof the metal foil 126 is achieved by laser ablating with the innerregion of the incident laser beam through at least a portion of themetal foil 126 at regions in alignment with locations between thealternating N-type and P-type semiconductor regions 104 and 106.

In either case, in an embodiment, impinging the incident laser beam onthe metal foil 126 involves generating a laser beam a laser cavity asalready having one of the above described beam shapes. In anotherembodiment, however, impinging the incident laser beam on the metal foil126 involves shaping an already generated laser beam to have one of theabove described beam shapes using optical diffraction.

As described above, in another embodiment, a metal layer 124 (that is, ametal seed) is not formed. As an example, FIG. 3 illustrates across-sectional views of another solar cell having foil-basedmetallization, in accordance with another embodiment of the presentdisclosure. Referring to FIG. 3, the metal foil 126 is laser welded(e.g., by laser welds 128) directly to the alternating N-type and P-typesemiconductor regions 104 and 106. In one such embodiment, in that casethat non-conductive material regions 122 are included, the metal foilcomes in direct contact with the non-conductive material regions 122.

Embodiments described herein include fabrication of a solar cellaccording to one or more of the above described approaches. Referring toFIGS. 1F and 3, in an embodiment, a solar cell includes a substrate 100.A plurality of alternating N-type 104 and P-type 106 semiconductorregions is disposed in (not shown) or above (as shown) the substrate100. A plurality of non-conductive material regions 122 may be includedin alignment with locations between the alternating N-type and P-typesemiconductor regions 104 and 106. A plurality of conductive contactstructures is electrically connected to the plurality of alternatingN-type and P-type semiconductor regions 104 and 106. Each conductivecontact structure includes a metal foil portion 126 disposed above andin alignment with a corresponding one of the alternating N-type andP-type semiconductor regions 104 and 106.

In a specific embodiment, referring particularly to FIG. 1F, eachconductive contact structure further includes a metal seed layer 124disposed directly between the corresponding one of the alternatingN-type and P-type semiconductor regions 104 and 106 and the metal foilportion 126. In another specific embodiment, referring particularly toFIG. 3, the metal seed layer 124 is not included and the metal foilportion 126 is welded directly to the corresponding one of thealternating N-type and P-type semiconductor regions 104 and 106.

As described above, a laser beam may be spatially shaped according tothe first exemplary embodiment of FIG. 1F. FIG. 4 is a plot 400 of laserintensity as a function of distance illustrating a cross-sectional viewof a spatially shaped laser beam having a beam shape with an innerregion of lower intensity and an outer region of higher intensity, inaccordance with an embodiment of the present disclosure.

Referring to FIG. 4, a laser beam profile 410 for a beam incident on ametal foil has an inner region 414 of lower intensity and an outerregion 412 of higher intensity. The inner region 414 and the outerregion 412 are taken as relative to a central axis 416 of the incidentlaser beam. In one such embodiment, the beam profile 410 has two outerregions 412, as is depicted in FIG. 4. In an embodiment, laser weldingof a metal foil to alternating N-type and P-type semiconductor regions(or to a metal M1 layer thereon) is achieved along pathway 402 using theinner region 414 of the incident laser beam 410. Patterning of the metalfoil is achieved at the same time as the laser welding by laser ablatingalong pathway 404 with the outer regions 412 of the incident laser beam410.

FIGS. 5A and 5B illustrate various processing operations in a method offabricating a solar cell using the laser beam profile of FIG. 4, inaccordance with an embodiment of the present disclosure.

Referring to part (i) of FIG. 5A, a M1 seed conductive layer is formedon N-type and P-type emitter regions above a solar cell substrate 500,i.e. on the device side of a solar cell, and is patterned or is formedas patterned. Referring to part (ii) of FIG. 5A, an M2 layer is locatedon the M1/cell pairing with direct contact maintained as suitable forlaser welding. Referring to part (iii) of FIG. 5A, a highly energeticbeam 502 is applied to locally heat the M2 layer and bond M1 and M2. Thespacing between weld spot may be optimized for the adhesion and thepatterning operation. In one embodiment, the highly energetic beam is along pulse duration, e.g., greater than 100 μs, laser or electron beam.Referring again to part (iii) of FIG. 5A, the inner portion of the laserbeam 502 is used to form a weld spot 504. The outer portion of the laserbeam 502 is used to weaken a M1 to device bond. In an embodiment, theouter portion of the laser beam 502 is used to cut the M2 layer whilethe inner region of the beam is used to weld the M2 layer.

Referring to part (iv) of FIG. 5B, in an optional embodiment, anotherlaser 510, e.g., of shorter pulse duration, less than 1 μs, is appliedto fabricate a guiding groove line for ultimate tear-off patterning. Inthat embodiment, referring to part (v) of FIG. 5B, one end of theunwelded foil may be gripped 512 and peeled off, such that the weakened(and laser grooved) portion is peeled off while leaving the weldedregion of the foil to remain on the cell.

As is described above, a laser beam may be spatially shaped according tothe second exemplary embodiment of FIG. 1F. FIG. 6 is a schematicillustrating a cross-sectional view of a spatially shaped laser beamhaving a beam shape with an inner region of higher intensity and anouter region of lower intensity, in accordance with an embodiment of thepresent disclosure.

Referring to part (a) of FIG. 6, a substrate 600 has a metal 1 (M1) andmetal 2 (M2) thereon. Referring to part (b) of FIG. 6, a laser beamprofile 610 for a beam incident on a metal foil (M2) has an inner region614 of higher intensity and an outer region 612 of lower intensity. Theinner region 614 and the outer region 612 are taken as relative to acentral axis 616 of the incident laser beam 610. In one such embodiment,the beam profile 610 has two outer regions 612, as is depicted in FIG.6. In an embodiment, laser welding (welding) of a metal foil (M2) toalternating N-type and P-type semiconductor regions (or to a metal M1layer thereon) is achieved using the outer regions 612 of the incidentlaser beam 610. Patterning (cutting) of the metal foil (M2) is achievedat the same time as the laser welding by laser ablating with the innerregion 614 of the incident laser beam 610.

As described briefly above, as an alternative to the embodiment depictedin FIG. 1F, initial yet incomplete patterning of a metal foil in contactwith emitter regions of a solar cell may be performed using a lasingablation process. The patterning is then completed in a subsequentprocess operation. As an example, FIGS. 7A and 7B illustratecross-sectional views of various stages in the fabrication of a solarcell using foil-based metallization, in accordance with anotherembodiment of the present disclosure.

Referring to FIG. 7A, a plurality of alternating N-type and P-typesemiconductor regions are formed above a substrate. In particular, asubstrate 700 has disposed there above N-type semiconductor regions 704and P-type semiconductor regions 706 disposed on a thin dielectricmaterial 702 as an intervening material between the N-type semiconductorregions 704 or P-type semiconductor regions 706, respectively, and thesubstrate 700. The substrate 700 has a light-receiving surface 701opposite a back surface above which the N-type semiconductor regions 704and P-type semiconductor regions 706 are formed.

In an embodiment, the substrate 700 is a monocrystalline siliconsubstrate, such as a bulk single crystalline N-type doped siliconsubstrate. It is to be appreciated, however, that substrate 700 may be alayer, such as a multi-crystalline silicon layer, disposed on a globalsolar cell substrate. In an embodiment, the thin dielectric layer 702 isa tunneling silicon oxide layer having a thickness of approximately 2nanometers or less. In one such embodiment, the term “tunnelingdielectric layer” refers to a very thin dielectric layer, through whichelectrical conduction can be achieved. The conduction may be due toquantum tunneling and/or the presence of small regions of directphysical connection through thin spots in the dielectric layer. In oneembodiment, the tunneling dielectric layer is or includes a thin siliconoxide layer.

In an embodiment, the alternating N-type and P-type semiconductorregions 704 and 706, respectively, are formed from polycrystallinesilicon formed by, e.g., using a plasma-enhanced chemical vapordeposition (PECVD) process. In one such embodiment, the N-typepolycrystalline silicon emitter regions 704 are doped with an N-typeimpurity, such as phosphorus. The P-type polycrystalline silicon emitterregions 706 are doped with a P-type impurity, such as boron. As isdepicted in FIG. 7A, the alternating N-type and P-type semiconductorregions 704 and 706 may have trenches 708 formed there between, thetrenches 708 extending partially into the substrate 700. Additionally,in one embodiment, a bottom anti-reflective coating (BARC) material 710or other protective layer (such as a layer amorphous silicon) is formedon the alternating N-type and P-type semiconductor regions 704 and 706,as is depicted in FIG. 7A.

In an embodiment, the light receiving surface 701 is a texturizedlight-receiving surface, as is depicted in FIG. 7A. In one embodiment, ahydroxide-based wet etchant is employed to texturize the light receivingsurface 701 of the substrate 700 and, possibly, the trench 708 surfacesas is also depicted in FIG. 7A. It is to be appreciated that the timingof the texturizing of the light receiving surface may vary. For example,the texturizing may be performed before or after the formation of thethin dielectric layer 702. In an embodiment, a texturized surface may beone which has a regular or an irregular shaped surface for scatteringincoming light, decreasing the amount of light reflected off of thelight receiving surface 701 of the solar cell. Referring again to FIG.7A, additional embodiments can include formation of a passivation and/oranti-reflective coating (ARC) layers (shown collectively as layer 712)on the light-receiving surface 701. It is to be appreciated that thetiming of the formation of passivation and/or ARC layers may also vary.

Referring again to FIG. 7A, a plurality of metal seed material regions714 is formed to provide a metal seed material region on each of thealternating N-type and P-type semiconductor regions 704 and 706,respectively. The metal seed material regions 714 make direct contact tothe alternating N-type and P-type semiconductor regions 704 and 706. Inan embodiment, the metal seed regions 714 are aluminum regions. In onesuch embodiment, the aluminum regions each have a thicknessapproximately in the range of 0.3 to 20 microns and include aluminum inan amount greater than approximately 97% and silicon in an amountapproximately in the range of 0-2%. In other embodiments, the metal seedregions 714 include a metal such as, but not limited to, nickel, silver,cobalt or tungsten. Optionally, a protection layer may be included onthe plurality of metal seed material regions 714. In a particularembodiment, an insulating layer 716 is formed on the plurality of metalseed material regions 714. In an embodiment, the insulating layer 716 isa silicon nitride or silicon oxynitride material layer. In anotherembodiment, in place of the metal seed regions 714, a blanket metal seedlayer is used that is not patterned at this stage of processing. In thatembodiment, the blanket metal seed layer may be patterned in asubsequent etching process, such as a hydroxide-based wet etchingprocess.

Referring again to FIG. 7A, a metal foil 718 is located with thealternating N-type and P-type semiconductor regions. In an embodiment,the metal foil 718 is an aluminum (Al) foil having a thicknessapproximately in the range of 5-100 microns. In one embodiment, the Alfoil is an aluminum alloy foil including aluminum and second elementsuch as, but not limited to, copper, manganese, silicon, magnesium,zinc, tin, lithium, or combinations thereof. In one embodiment, the Alfoil is a temper grade foil such as, but not limited to, F-grade (asfabricated), O-grade (full soft), H-grade (strain hardened) or T-grade(heat treated). In one embodiment, the aluminum foil is an anodizedaluminum foil.

It is to be appreciated that, in accordance with another embodiment ofthe present disclosure, a seedless approach may be implemented. In suchan approach, metal seed material regions 714 are not formed, and themetal foil 718 is located with the material of the alternating N-typeand P-type semiconductor regions 704 and 706. For example, in oneembodiment, the metal foil 718 is located directly with alternatingN-type and P-type polycrystalline silicon regions.

Referring to FIG. 7B, the metal foil 718 is directly coupled with acorresponding portion of each of the metal seed material regions 714. Inone such embodiment, the direct coupling of portions of the metal foil718 with a corresponding portion of each of the metal seed materialregions 714 involves forming a metal weld 720 at each of such locations,as is depicted in FIG. 7B. In an embodiment, the metal foil 718 isadhered directly to the plurality of metal seed material regions 714 byusing a laser welding process. In an embodiment, the optional insulatinglayer 716 is included, and adhering the metal foil 718 to the pluralityof metal seed material regions 714 involves breaking through regions ofthe insulating layer 716, as is depicted in FIG. 7B.

FIG. 7B further illustrates the structure of FIG. 7A following formationof laser grooves in the metal foil. Referring again to FIG. 7B, themetal foil 718 is laser ablated through only a portion of the metal foil718 at regions corresponding to locations between the alternating N-typeand P-type semiconductor regions 704 and 706, e.g., above trench 708locations as is depicted in FIG. 7B. The laser ablating forms grooves730 that extend partially into, but not entirely through, the metal foil718. In an embodiment, forming laser grooves 730 involves laser ablatinga thickness of the metal foil 718 approximately in the range of 80-99%of an entire thickness of the metal foil 718. That is, in oneembodiment, it is critical that the lower portion of the metal foil 718is not penetrated, such that metal foil 718 protects the underlyingemitter structures. In an embodiment, the laser welding and the laserablation of FIG. 7B are performed at the same time using a spatiallyshaped laser as described above.

In a first exemplary embodiment, the remaining metal foil 718 issubsequently anodized at exposed surfaces thereof to isolate regions 740of the remaining metal foil 718 corresponding to the alternating N-typeand P-type semiconductor regions 704 and 706. In particular, the exposedsurfaces of the metal foil 718, including the surfaces of the grooves730, are anodized to form an oxide coating. At locations correspondingto the alternating N-type and P-type semiconductor regions 704 and 706,e.g., in the grooves 730 at locations above the trenches 708, the entireremaining thickness of the metal foil 718 is anodized there through toisolate regions 740 of metal foil 718 remaining above each of the N-typeand P-type semiconductor regions 704 and 706.

In a second exemplary embodiment, instead of anodizing the metal foil718 to isolate portions of the metal foil 718, the patterned metal foil718 is etched to isolate portions 740 of the metal foil 718. In one suchembodiment, the structure of FIG. 7B is exposed to a wet etchant.Although the wet etchant etches all exposed portions of the metal foil718, a carefully timed etch process is used to break through the bottomsof the laser grooves 730 without significantly reducing the thickness ofthe non-grooved regions 740 of the metal foil 718. In a particularembodiment, a hydroxide based etchant is used, such as, but not limitedto, potassium hydroxide (KOH) or tetramethylammonium hydroxide (TMAH).

Although certain materials are described specifically with reference toabove described embodiments, some materials may be readily substitutedwith others with other such embodiments remaining within the spirit andscope of embodiments of the present disclosure. For example, in anembodiment, a different material substrate, such as a group III-Vmaterial substrate, can be used instead of a silicon substrate.Additionally, although reference is made significantly to back contactsolar cell arrangements, it is to be appreciated that approachesdescribed herein may have application to front contact solar cells aswell. In other embodiments, the above described approaches can beapplicable to manufacturing of other than solar cells. For example,manufacturing of light emitting diode (LEDs) may benefit from approachesdescribed herein.

Thus, approaches for foil-based metallization of solar cells and theresulting solar cells have been disclosed.

Although specific embodiments have been described above, theseembodiments are not intended to limit the scope of the presentdisclosure, even where only a single embodiment is described withrespect to a particular feature. Examples of features provided in thedisclosure are intended to be illustrative rather than restrictiveunless stated otherwise. The above description is intended to cover suchalternatives, modifications, and equivalents as would be apparent to aperson skilled in the art having the benefit of this disclosure.

The scope of the present disclosure includes any feature or combinationof features disclosed herein (either explicitly or implicitly), or anygeneralization thereof, whether or not it mitigates any or all of theproblems addressed herein. Accordingly, new claims may be formulatedduring prosecution of this application (or an application claimingpriority thereto) to any such combination of features. In particular,with reference to the appended claims, features from dependent claimsmay be combined with those of the independent claims and features fromrespective independent claims may be combined in any appropriate mannerand not merely in the specific combinations enumerated in the appendedclaims.

What is claimed is:
 1. A solar cell, comprising: a plurality of N-typeand P-type semiconductor regions disposed above a substrate; a metallayer disposed on the N-type and P-type semiconductor regions, the metallayer disposed on and in alignment with the N-type and P-typesemiconductor regions; and a metal foil disposed on and bonded to themetal layer, wherein the bond between the metal foil and the metal layercomprises weld regions, the plurality of weld regions approximatelyequally spaced from a border between the nearest N-type and P-typesemiconductor regions.
 2. The solar cell of claim 1, wherein the metallayer comprises a metal seed layer.
 3. The solar cell of claim 1,wherein the metal layer comprises an aluminum layer.
 4. The solar cellof claim 1, wherein the meta foil comprises an aluminum foil.
 5. Thesolar cell of claim 1, wherein the N-type and P-type semiconductorregions are N-type and P-type regions in a polycrystalline silicon layerdisposed above the substrate.
 6. The solar cell of claim 1, furthercomprising a thin dielectric layer disposed on the substrate, the thindielectric layer disposed between the N-type and P-type semiconductorregions and the substrate.